Fruit flies (Drosophila melanogaster and other spp). If you don't have an inside track at a biology lab your initial culture will have to come from one of the suppliers listed at the live food cultures page, because wild fruit flies are far too lively and adventurous to culture for fish food. The mutants with vestigial wings stick closer to home, to the relief of your family. They still hop like crazy fleas. A few minutes in the fridge tranquilizes them, and then you can tap them from their culturing vial right into the filter outlet flow. Top-feeding fish just go bananas. "Wild!" I murmur dramatically, like the familiar tv Wild Discovery voice-over.
I enjoy the neatness of the fruit-fly culturing kits that are composed of a clear plastic vial with ventilated snap-on lid, foam plug and a strip of stiff plastic netting. Your supplier will supply you.
Otherwise, use the kind of pickling jar that has a lid insert nested in the threaded screw-on metal rim, with the insert replaced by a square of nylon stocking, over which you lightly screw down the lid. Escaped flies are less disastrous than wild flies getting in among your wingless slaves. The genes for normal wings are dominant, and your vestigial-winged Drosophila will be flying high in a single generation. Even on their own a vial of fruit flies can mutate back to the winged wild form; if this happens, freeze the vial and destroy the contents, which your fish will enjoy, both the adult flies and the larvae, which you'll need to soak and rinse, to free them of the culture medium.
Fruit flies produce a new generation from eggs in about ten days. The females deposit tiny white eggs which hatch in a day or so to a white larva that spends about a week eating its way through the pasty medium before climbing out to pupate on the walls of the jar. If the pupae keep close to the surface of the medium, it's a sign that the humidity isn't high enough. I've killed flies by letting them get too dry, but I've killed more by putting them into new culture vials that were still wet on the inside surfaces. Two things interfere with successful fruit fly culture: mold and mites. Each can be controlled, with mold inhibitors like Moldaway and mite-suppressors like Tedion.
Links. The fruit fly is one of the "model organisms" in genome research and other aspects of biology, so major information on all aspects of Drosophila is in the biosciences section at the WWW Virtual Library.
Culturing links. Julian Haffegee's very thorough illustrated 1999 article on Drosophila culture is now archived as a complete fruit fly section at the killifish site Killi.co.uk.
Tree frogs also eat fruit flies, so the British Dendrobatid Group homepage carries a fine article on fruit flies and their general culture— plus lots of stuff on Dendrobates and related tree frogs.
Many of the live food culture suppliers that have their own page here will mail you a starter kit of fruit flies. The Drosophila Company specializes in these critters.